The plain format implementation resides in this module. The plain format
does not use any hashing and basically just wraps a normal filesystem tree
in the hashed-storage API.

NB. The read function on Blobs coming from a plain tree is susceptible to
file content changes. Since we use mmap in read, this will break
referential transparency and produce unexpected results. Please always make
sure that all parallel access to the underlying filesystem tree never
mutates files. Unlink + recreate is fine though (in other words, the
writePlainTree and plainTreeIO implemented in this module are safe in
this respect).

Run a TreeIO action in a plain tree setting. Writes out changes to the
plain tree every now and then (after the action is finished, the last tree
state is always flushed to disk). XXX Modify the tree with filesystem
reading and put it back into st (ie. replace the in-memory Blobs with normal
ones, so the memory can be GCd).